The demand for improved coatings and powder particle products in the thermal spray industry has been relentless as the technology suffers from compositional non-homogeneity of injected solution particles. One solution to achieve homogeneity in coatings and particle products is aimed at repeatedly producing uniform droplets with uniform diameter. Precise control of the size of the solution droplets injected into a thermal spray system achieves more precise control of the particle melt for successful and improved coatings and powder generation. The methods of droplet generation using capillary streams involve the use of a piezoelectric device impinging a pressure pulse on the walls of a reservoir vessel full of a liquid solution. In general, one such method is the imposition of amplitude modulated sinusoidal carrier disturbance on the piezoelectric device. These methods generally involve piezoelectric devices (“piezo”) in direct contact with the liquid source. One method involves using an oscillating crystal in direct contact with a liquid source to impart a disturbance and initiate capillary instability responsible for stream break up into droplets. The disturbance is imposed in a compressive fashion at the top of the liquid volume and propagated downstream to the capillary nozzle. Another method imparts this disturbance on the side wall of a columnar liquid contained in a radially contracting piezoelectric cylinder that forces liquid through a capillary nozzle and is said to produce uniform stream of droplets. These droplet generation methods are, in general, limited to high droplet diameter and/or work at frequencies no higher than 10 KHz.
Applications of droplet apparatuses known in the art have the piezo in direct contact with the liquid. For example, in a typical printer design, the piezo is immersed in the printing liquid and serves as a gate to allow or forbid droplet exit as the piezo stretches or contracts under electrical drive. In another application, the piezo oscillations are transmitted directly to the liquid so that the piezo is in contact with the liquid or, if not in contact, the transmission is done through an elastic membrane. Furthermore, the effect of oscillations involves only a small volume of liquid directly near the nozzle.